Dryer section of a paper maschine

Abstract: The invention concerns a device in the drying of a paper web (W), wherein the paper web (W) is on support of a drying wire (20 i ) without long open draws of the web (W). The paper web (W) is contact-dried by pressing it with the drying wire (20) onto the cylinder face (21'), whose diameter is D 2 >1.5 m, on a sector b, whose magnitude is b>180°. The web (W) is evaporation-dried as blowing-on-drying by means of high-velocity (v 9 ) drying-gas jets applied to the web (W) on the drying wire (20) on the face of the following large-diameter D 1 >2 m cylinder (15) on a sector a>180° while the web (W) is on the side of the outside curve. The web (W) to be dried is passed over the sector c of the suction roll (22), which sector c is subjected to negative pressure, while the web (W) is supported on the drying wire (20) at the side of the outside curve, the magnitude of said sector being c>160°, and the diameter D 3 of said suction roll (22) being D 3 <D 2 .

Description: The invention concerns a dryer section of a paper machine, in particular of a high-speed paper machine, whose running speed is v ≈ 25...40 metres per second, wherein the dryer section comprises plural drying modules placed one after the other. The pre-characterizing clause of claim 1 is based on document FI-B-87669. The highest web speeds in paper machines are currently already of an order of 25 metres per second, but, before long, the speed range of 25...40 m/s will be taken into use. Then, a bottle-neck in the runnability of a paper machine will be the dryer section, whose length with the use of the prior-art multi-cylinder dryers would additionally become intolerably long. If a present-day multi-cylinder dryer were operated at a web speed of 40 m/s, it would comprise about 70 drying cylinders, and its length in the machine direction would be about 180 m. In such a case, the dryer would comprise about 20 different wire groups and a corresponding number of group-gap draws. It can be assumed that, in a speed range of 30...40 m/s, the runnability of normal prior-art multi-cylinder dryers would not be even nearly satisfactory, but there would be a great number of web breaks, which lowers the efficiency of the paper machine. In a speed range of 30...40 m/s and at higher speeds, the prior-art multi-cylinder dryers would also become uneconomical, because the investment cost of an excessively long paper machine hall would become unduly high. It can be estimated that the cost of a paper machine hall is currently typically about 1 million FIM per metre in the machine direction. In a paper machine hall, space is usually available in the direction of height, and so it has been suggested that the cylinders in a multi-cylinder dryer be arranged in vertical stacks, but in such a case, especially with high speeds, the problems of runnability and broke removal are emphasized and are likely to be very difficult to solve in the speed range of 30...40 m/s. With respect to this prior art, reference is made to document EP-A-0 383 744 corresponding to document FI-B-82097. One parameter that illustrates the drying capacity of the prior-art multi-cylinder dryers is the amount of water evaporated in the dryer section per unit of floor area covered by the web to be dried, in a unit of time. In the prior-art multi-cylinder dryers, this parameter is typically in the range of 50...80 kilograms of H 2 O per square metre in an hour (kg H 2 O/m 2 /h). It is known in prior art to use various blowing-on/blowing-through units for evaporation drying of the paper web, which units have been used in particular in the drying of tissue paper. With respect to this prior art, reference is made by way of example to the following documents: US-A-3,301,746, US-A-3,418,723, US-A-3,447,247, US-A-3,541,697, US-A-3,956,832, US-A-4,033,049, CA-B-2,061,976, FI-B-57,457, and FI-B-87669. The paper web dryer described in document US-A-4,033,048 is not suitable for use at the high speeds of v > 25 m/s meant in the present invention, and in particular not at a speed range of v ≈ 30...40 m/s or higher. In this respect and in other respects, the solution of said document has the following drawbacks. According to said document a suction box is fitted inside the support-fabric loop, by means of which box both a large suction roll and the pocket placed underneath the suction roll and between the outside heated rolls are subjected to negative pressure. In such a case, a problem consists of the lateral seals, through which significant amounts of air leak. The leakage air again produces a strong air current in the transverse direction of the machine in the lateral areas of the web, which deteriorates the stable run of the web through the dryer and, consequently, the runnability and the efficiency of the whole machine. Owing to the large amount of leakage air, the subjecting of the pocket and of the large suction roll to the level of negative pressure that is necessary at high speeds in order to ensure a stable run of the web requires large air ducts and blowers and, therefore, consumes a lot of energy. The object of the present invention is to provide novel solutions for the problems discussed above. The principal object of the invention is to provide a novel dryer section ,which is suitable for use at high web speeds of v > 25 m/s, said speeds being most appropriately of an order of v ≈ 30...40 m/s or even higher. It is an object of the present invention to provide novel solutions of drying for the speed range mentioned above so that, in spite of the very high web speed, the runnability of the dryer section can be kept at a satisfactory level. It is an object of the present invention to provide a dryer section in which both contact drying on a drying cylinder and blowing-on drying are applied in a novel synergic way. It is a further principal object of the present invention, by means of a blowing-on flow to increase the drying speed and thereby to make the dryer section shorter, which contributes to an improved runnability of the dryer. It is a further object of the invention to provide a dryer section by whose means, at said high speed range, the length of the dryer section in the machine direction, yet, becomes reasonable so that its length does at least not become substantially longer than the length of the prior-art cylinder dryers. If this goal can be achieved, that would permit renewals and modernizations of paper machines in existing paper machine halls up to a web speed of v ≈ 40 m/s and even higher. It is a further object of the invention to provide a dryer section , in which the web is reliably fixed to the drying wire over the entire length of the dryer section so that transverse shrinkage of the web can be substantially prevented, whereby transverse non-homogeneity of the web, arising from an uneven transverse shrinkage profile, can be avoided. In view of achieving the objectives stated above, the dryer section of the invention comprises the features of claim 1. The scope of the invention also includes such dryer sections in which drying modules of the kind as defined in claim 1 are employed at suitable locations together with prior-art cylinder groups, in particular together with so-called "normal" cylinder groups provided with single-wire draw, in which groups the drying cylinders are placed in the upper row and the reversing suction rolls in the lower row, or the other way round. Between said groups and said drying modules preferably closed group-gap draws are employed. For a dryer section of this kind, the designation "hybrid dryer section" is used. In the invention, as the drying gas, preferably either air or superheated steam is used. The state of the drying gas is chosen at each drying stage in consideration of the way in which the water is bound to the fibre mesh of the paper web at each particular drying stage. In this way, a drying process is provided that is optimal both in view of the paper quality and in view of the drying. In the dryer section in accordance with the invention, as a blowing-on drying cylinder and as a reversing suction roll, most advantageously, such drying cylinders and reversing suction rolls provided with grooved and perforated mantles can be used as are marketed by the applicant under the trade mark VAC™ roll and whose details come out from document FI-B-83,680 (equivalent to US-A-5,022,163). As the web is preferably kept firmly in contact with the drying wire over the entire length of the dryer section while on the curved sectors on which the web remains outside, employing a differential pressure, transverse shrinkage of the web during drying is prevented, whereby transverse non-homogeneity of the web, arising from an uneven transverse shrinkage profile, is eliminated. In the dryer section in accordance with the invention, the drying hood of the blowing-on drying cylinder can be divided into a number of blocks in the transverse direction of the machine by means of walls placed in the machine direction, into which blocks drying gases of different temperature, humidity and/or pressure are passed or in which blocks sets of drying-gas jets of different velocities are employed. In this way, the drying of the paper web can be regulated in the transverse direction, and a favourable moisture profile can be obtained, which has a certain form, usually uniform, in the transverse direction. In a preferred embodiment of the dryer section in accordance with the invention, between two blowing-on cylinders (large cylinders) placed inside the same support-fabric loop, there are two contact-drying cylinders and a reversing suction roll of smaller diameter between them. This comes from practical limitations of constructing a blowing-on hood of maximally large covering area around a cylinder at the same time as it is desirable to obtain a maximally efficient support for the web between said blowing-on cylinders. In the present invention, it is expressly essential that, if hot air is used as the medium, said air has a considerable velocity against the web in the blowing-on drying. According to the invention, in the first drying module or modules, larger diameters of large cylinders and contact-drying cylinders are employed than in the later drying module or modules, in which latter ones it is preferable to employ such diameters of large cylinders and contact-drying cylinders as well as of reversing suction rolls as have been chosen as optimal in view of the quality of the paper to be produced and in view of the machine construction. By means of the large cylinder diameters of the first drying module or modules, in the initial part of the dryer section, on the different cylinders, drying energies higher than average and longer dwell times of the web become available, and thereby quantities of water larger than average can be evaporated per unit of length of the dryer in the machine direction. In this way, in the initial part of the dryer section, the dry solids content and the strength of the web can be raised rapidly to such a level that a reliable transfer of the web can be accomplished, also by means of open draws of the web if necessary. Moreover, when said larger cylinder diameters are employed the centrifugal forces that tend to separate the web from the drying wire can be made lower, for which reason it is also possible to employ lower levels of negative pressure at said cylinders, which is advantageous both in view of the cost of equipment and in view of the cost of energy. The dryer section in accordance with the present invention is also particularly well suitable for modernization of existing dryer sections. In such a case, the procedure can be, for example, such that over a part of the length of the dryer section in the initial end of the dryer section, several drying modules are provided, whose drying capacity per unit of length in the machine direction is higher than in the dryer section to be modernized on the average. After said drying modules, it is possible to use the existing multi-cylinder dryer, which comprises preferably several wire groups. In such a case, for example, such a final end of a dryer section is advantageous as comprises exclusively groups with single-wire draw, either so-called normal groups and, between them, corresponding inverted groups, or exclusively so-called normal groups. In such a case, one or two last group(s) may consist of a group with twin-wire draw in which the web has free draws between the rows of contact-drying cylinders, on which draws the web can be relaxed. As the last group or groups, it is possible to use a group with twin-wire draw, because, at that point, the web is sufficiently dry and strong so that the free draws of the web do not produce a detrimental risk of web break. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing. Figure 1 is a schematic side view of the whole of a dryer section. Figure 2 shows a preferred geometry of a drying module as well as the most important parameters of dimensioning and a combination of evaporation means consisting of three successive modules. Figure 3 shows drying modules in which there are two drying hoods connected together. Figure 4 shows a modification in which there are straight joint runs of the drying wire and the web between the contact-drying cylinders and the blowing-on-drying cylinders and the reversing suction rolls. Figure 5 illustrates arrangements of circulation of the drying gas in connection with the drying hood of a blowing-on drying cylinder. Fig. 6 is a schematic illustration of an exemplifying embodiment of the arrangement of circulation of the drying gases and blow airs. Figure 7 is a sectional view taken along the lines XI-XI in Figs. 5 and 6. Figure 8 shows an embodiment of the dryer section in accordance with the invention in which cylinder diameters larger than average are employed in the first drying module. To begin with, an example of the construction of a drying module 10 will be described mainly with reference to Figs. 2 and 5. The drying module 10 comprises a large-diameter D 1 blowing-on drying cylinder 15, for which the designation "large cylinder" will be used in the following. The mantle 16 of the large cylinder 15 is provided with through perforations and/or with grooves in its outside face (Fig. 7), in which grooves 16R negative pressure can be produced through the perforations in the mantle 16 or otherwise to keep the web W on the face of a drying wire 20 on a sector a. The drying module 10 includes contact-drying cylinders 21 placed at the proximity of the large cylinder 15 at both of its sides, which contact-drying cylinders 21 have a smooth outer face 21' heated from the interior. For these cylinders 21, in the following, the designation "contact cylinder" will be used, because the web W to be dried is pressed by the drying wire 20 into direct contact against them, whereas, on the sector a of the large cylinder 15, the web W is placed on the drying wire 20 with the drying wire 20 being located between the large cylinder 15 and the web w. Further, the drying module 10 includes one reversing suction roll 22, which is provided with through perforations. In the following, the designation "suction roll" will be used for this roll 22. Said large cylinder 15 and said suction roll 22 are most appropriately VAC™ rolls described in document FI-B-83,680 (equiv. to US -A- 5,022,163) or equivalent, which are provided with perforations 16P passing through the roll mantle 16 or 23, respectively and opening into the grooves 16R in the outer face of the roll mantle (Fig. 7). In said grooves 16R, negative pressure is produced from the negative pressure p o present in the interior of the mantles 16;23 of the large cylinder 15 and of the suction roll 22, which negative pressure p o is again produced through a suction duct 18;38a placed in an axle journal of the large cylinder 15 and of the suction roll 22 by means of a vacuum pump 37;38 (Figs. 5 and 6). Further, the drying module 10 includes the drying wire 20, which is guided by guide rolls 25. The permeability of the wire, i.e. the penetrability by air, is chosen as suitable in view of the invention, and in successive different drying wires it is possible to use different permeabilities and different wire tensions in the machine direction. In the module 10 in the first step, the paper web W is dried by pressing it by means of the drying wire 20 against the cylinder face 21', whose diameter is chosen as D 2 > 1.5 m, on a sector b, whose magnitude b > 180°. In the next step, the paper web W is evaporation-dried by means of blowing-on drying by means of a set of high-velocity v 9 ≈ 50...150 m/s drying-gas jets on support of the drying wire 20 on the face of the large cylinder 15, whose diameter is chosen as D 1 > 2 m, while the web W is supported on the drying wire on the sector a > 180° preferably over the area of the whole sector a. Hereupon the first step defined above is repeated. Before said first step and/or after the last-mentioned step, the web W to be dried is passed over a sector c of the suction roll 22 while the web W is on support of the drying wire 20 with the drying wire 20 being located between the suction roll 22 and the web w. The magnitude of the sector c is chosen as c > 160°, and the diameter of the suction roll 22 is chosen as D 1 < D 2 . The velocity range of the set of drying-gas jets that is used is preferably v g ≈ 80...130 m/s. The diameters of the cylinders and rolls 15,21,22 and 25 mentioned above are denoted with D 1 ,D 2 ,D 3 and D 4 , respectively. In the drying module 10 , preferably, D 1 > D 2 > D 3 > D 4 . Moreover, it is advantageous that the ratios D 1 /D 2 and D 2 /D 3 should be chosen within the following ranges: D 1 /D 2 ≈ 1.0...2.2, preferably D 1 /D 2 ≈ 1.5...1.7, D 2 /D 3 ≈ 1.1...2.2, preferably D 2 /D 3 ≈ 1.2...1.6, and D 3 /D 4 ≈ 1.0...2.5, preferably D 3 /D 4 = 1.5...2.0. The drying module 10 is as compact as possible especially in the horizontal direction, i.e. in the machine direction, and its horizontal dimensions 1 10 and 1 11 indicated in Fig. 2 are preferably chosen as follows: 1 11 = (0.8...4.0) x D 1 , preferably 1 11 = (1.8...3.0) x D 1 , and the height dimensions h 1 and h 2 are chosen preferably so that h 2 = (0.1...1.1) x D 2 , and h 1 /h 2 ≈ 2...10, preferably h 1 /h 2 ≈ 3...6. In the module 10 , the sectors of turning of the drying wire 20 and of the web W on the rolls 15 and 21 are chosen preferably so that a ≈ 180°...320°, preferably a ≈ 220°...300°, b ≈ 180°...300°, preferably b ≈ 210°... 260°, and the turning sector c of the web W on the suction roll 22 (in Fig. 2) between the modules 10 2 and 10 3 is c ≈ 160°...300°, preferably c ≈ 200°...270°. Fig. 1 shows a dryer section of a paper machine that consists of drying modules 10 described above, which dryer section is intended typically for a web speed of 30...40 m/s. The dryer section shown in Fig. 1 does not comprise all of the features of the invention. The whole of the dryer section is placed inside a hood 100. The paper web W is passed into the interior of the hood 100 in the direction of the arrow W in through the opening 103 in the hood 100, being removed out of the hood 100 at the final end of the dryer section through the opening 104 in the direction of the arrow W out . In the hood 100, in a way in itself known, air-conditioning is provided, which is illustrated by the air inlet duct 105, through which dry and possibly heated air is passed through the nozzles 101 and 101a and 101b into the hood. Out of the hood 100, air is removed through the ducts 106a and 106b. The exhaust-air flows are produced by means of the blowers 102a and 102b. The moist air is removed in the direction of the arrows A out into the open air through a heat recovery equipment. According to Fig. 1, in the direction of arrival of the web W in , the dryer section comprises two "inverted" drying modules 10 1 and 10 2 , in which the large cylinders 15 and drying hoods 11 associated there with are placed underneath and the pairs of contact cylinders 21 are placed above. The inverted modules 10 1 , 10 2 have a common drying wire 20 1 , which carries the web W as a fully closed draw through the group comprising the inverted modules 10 1 , 10 2 , whereupon the web W is transferred as a closed draw C 1 onto the drying wire 20 2 of a following group comprising "normal" module 10 3 , 10 4 , from which wire the web is transferred as a closed draw C 2 onto the drying wire 20 3 of a following group comprising inverted modules 10 5 ,10 6 . From the drying wire 20, the web W is transferred as a closed draw C 3 on the drying wire 20 4 of the last "normal" group comprising modules 10 7 ,10 8 . In Fig. 1, the overall length of the dryer section is denoted with L 1 . Typically, the length of a dryer section as shown in Fig. 1 is L 1 ≈ 40...60 m. According to what has been stated above, the evaporation speed per unit of length x width, i.e. per floor area covered by the web to be dried, which speed illustrates the compactness of the dryer section, i.e. the efficiency of utilization of the longitudinal space, is 100...160 kg H 2 O/m 2 /h. In Fig. 1, about 75...80 per cent of the length of the web W is either on the sectors a of the large cylinders 15 as subject to a blowing-on drying effect or on the cylinders 21 as subject to a drying effect of the contact-drying face. By means of the modules 10 , it is possible to form various so-called hybrid dryers, sections. There are one or several modules 10 at suitable locations, and, moreover, in a hybrid dryer section, there are groups of drying cylinders, preferably such "normal" groups R in which the contact-drying cylinders 21a are placed in the upper row and the reversing suction rolls 22 in the lower row, but, if necessary, it is also possible to use so-called inverted groups, even though in them, when breaks occur, difficulties are encountered in the handling of paper broke. The most important dimensioning parameters of the construction of a group of modules 10 1 , 10 2 , 10 3 as shown in Fig. 2 were already described above. In Fig. 2, the first module 10 1 is a so-called inverted module, in which the large cylinder 15 is placed underneath and the pair of contact cylinders 21 above. The web W is transferred from the face of the drying wire 20 onto the face of the wire 20 2 , which runs over the first suction roll 22 in the module 10 2 , on the sector c o . Hereupon the web W is transferred on the suction roll 22, being held by the negative pressure present in the grooves 16R in the roll mantle (Fig. 7), onto the next contact cylinder 21, against whose heated smooth face the web is pressed by the effect of the tension of the wire 20 2 on the sector b. Hereupon the web W is transferred substantially directly onto the grooved face of the large cylinder 15, on which face it is held by the effect of the negative pressure present in the grooves 16R. The drying sector a of the large cylinder 15 is as large as possible, preferably a ≈ 300°. After the sector a, the web W is transferred substantially directly onto the next contact-drying cylinder 21, and after its maximally large drying sector b, preferably b ≈ 270°, being transferred by the reversing suction roll 22, to the next drying module 10 3 . Fig. 3 shows such a pair of drying modules 10 1 , 10 2 in which the drying hoods 11 1 and 11 2 of both of them are divided by partition walls 12 1 and 12 2 into two compartments 10a and 10b. The pair of drying hoods 11 1 ,11 2 of the drying module 10 1 ,10 2 has a common vertical partition wall 12, which runs at, or in the area of, the centre of rotation of the contact cylinder 21 placed underneath. Fig. 4 shows such a drying module 10 in accordance with the invention in which the drying wire 20 and the web W have relatively short straight draws 20S between the large cylinder 15 and the contact cylinders 21. Between the contact cylinders 21 and the suction roll 22, the drying wire 20 has also very short straight draws 20S o . In the areas of said straight draws 20S;20S o , it is possible to arrange ejection blow boxes 13 in themselves known, by means of whose air blowings induction of pressures in the closing nip spaces N+ is prevented, for, in the contrary case, said pressures would cause separation of the web W from the drying wire 20 at the nips N+. Besides the blowing-on drying cylinder, the drying module shown in Fig. 4 comprises three contact cylinders 21 and two reversing suction rolls 22. Fig. 5 shows the construction of the drying hood 11 placed around the large cylinder 15 and the arrangement of circulation of the drying gas, such as air or superheated steam. The hood 11 is divided by a partition wall 12 into two compartments 10a and 10b. The hot drying gas is passed into the compartments 10a,10b through feed pipes 30, from which the drying air is distributed through a duct 41 into a nozzle chamber 40, which is defined from outside by the curved wall 42 and from inside by a nozzle field 43, which is placed at the distance of a very small gap, Δ ≈ 10...60 mm, preferably Δ ≈ 20...30 mm, from the outer face of the web W running on the drying wire 20. The large cylinder 15 is provided with the mantle 16 with through perforations 16P and outside grooves 16R, the through perforations 16P opening into said grooves (Fig. 7). The interior of the large cylinder 15 communicates with a suction pipe 19 through a suction duct 18 placed in connection with a support 17 of an axle journal of said cylinder, which suction pipe 19 communicates with a suction pump 37 (Fig. 6) so as to produce a negative pressure p o ≈ 0.5...20 kPa in the grooves 16R in the mantle 16. On the sector a of the large cylinder 15, the web is subjected to a differential pressure ΔP 1 , which presses the web W to be dried against the drying wire 20 while the web W is placed at the radially outer side of the drying wire and tends to be separated from the drying wire 20 by the effect of centrifugal forces, which forces are proportional to the factor 2 v 2 /D 1 . These separating forces are counteracted by means of the differential pressure ΔP 1 , which is effective between the outer face of the web and the grooves 16R in the mantle 16 of the large cylinder 15. This differential pressure ΔP 1 is, as a rule, chosen in the range of ΔP 1 = 1...4 kPa. For a corresponding purpose, on the sector c of the reversing suction rolls 22, on which sector the web W is placed at the radially outer side of the drying wire, a difference in pressure ΔP 2 is used, which is, as a rule, chosen in the range of ΔP 2 = 1...4 kPa. These differences in pressure ΔP 1 and ΔP 2 are produced by means of negative pressure passed into the interior of the large cylinder 15 and the reversing suction roll 22 through a suction duct 18;38a placed in connection with the axle journal of the cylinder or roll, which negative pressure also produces the leakage flows F 1 ,F 2 outside the sectors a and c, to be described in the following. As is indicated in Figs. 2, 5 and 6, on the sector 360°-a of the large cylinder 15, i.e. on the sector that is not covered by the drying wire 20, a leakage flow F 1 takes place through the cylinder mantle 16 towards the interior of the cylinder, but, by means of suitable dimensioning of the throttle in the through perforations 16P, i.e. of the resistance to flow, this leakage flow F 1 can be brought to such a level that it does not disturb the formation of a sufficient differential pressure ΔP 1 in the grooves 16R. A corresponding leakage flow also takes place on the free sectors 360°-c of the suction rolls 22, and this flow is denoted with F 2 in Figs. 2 and 6. The large cylinder 15, and so also the reversing suction rolls 22, may also be provided with inside suction boxes and sealing members to minimize said leakage flows. Fig. 6 is a schematic illustration of an exemplifying embodiment of the arrangement of circulation of the drying gases and blow airs. Into the compartments 10a and 10b of the drying hood 11, the inlet flows B in are passed through the feed pipes 30. The state of the inlet gas passed into different compartments 10a and 10b may be different. The inlet flows B in are regulated by means of regulation dampers 31. From the nozzle field 43, the high-energy hot drying-gas flows are applied at a high velocity v 9 = 50...150 m/s to the outer face of the web W, whereby so-called blowing-on drying or "impingement" drying is produced. According to Fig. 6 , air blowings of ejection blow boxes 13 are passed out of blower 36 in the direction of the arrows B 3 , by means of which blowings the formation of pressure in the closing nip spaces N+ is prevented. One of the axle journals of the suction rolls 22 includes the suction duct 38a, through which, in the direction of e arrows B 5 , a suction flow is passed out of the interior spaces in the suction rolls 22 by means of a suction pump 38. In this way, on the outer face of the perforated 16P and grooved mantle 23 of the suction roll 22, negative pressure is produced, by whose means the web W is held in connection with the suction roll 22 and the drying wire 20 as it runs on the sectors c at the radially outer side of the drying wire. Further, Fig. 6 shows that a replacement air flow is passed by means of a blower 39 in the direction of the arrow B 4 through a duct 14 to constitute replacement air for the hood 100. The duct 14 corresponds to the blow nozzles 101 shown in Figs. 1 . Fig. 7 shows axial sectional views of the mantle 16;23 of the large cylinder 15 and the reversing suction roll 22 taken along the lines XI-XI in Figs. 5 and 6. Said mantles 16;23 are provided with the annular grooves 16R passing around their outer faces, the depth of said grooves being denoted with r o and the width of the grooves with l o as well as the mantle portions of full wall thickness, placed between the grooves, with l 1 . The perforations 16P that pass through the mantle 16;23 are opened into the bottoms of the grooves 16R. The diameter of the holes is denoted with , and the full thickness of the mantle 16;23 with r 1 . In the following, a preferred example of dimensioning of a grooved mantle as shown in Fig. 7 will be given: r o ≈ 5 mm, l o ≈ 5 mm, r 1 ≈ 20 mm, l 1 ≈ 15 mm,  ≈ 4 mm. The frequency of the perforations 16P and the diameters are preferably chosen so that the percentage of holes in the total area of the groove 16R bottoms is about 1...3 per cent. Fig. 8 shows a dryer section according to a preferred embodiment of the invention in which, in the first two drying modules 10 1 and 10 2 , placed in the initial part of the dryer section, larger diameters D 1A , D 2A and D 3A of the cylinders 15A and, 21A and of the rolls, 22A are used as compared with the following two drying modules 10 3 and 10 4 , in which the corresponding diameters are denoted with D 1 , D 2 and D 3 . The first drying modules 10 1 and 10 2 have a common drying wire 20 1 , and, in a corresponding way, the following two drying modules 10 3 and 10 4 have a common drying wire 20 2 . By means of the cylinder diameters D 1A , D 2A and D 3A larger than average, the web W to be dried can be given longer dwell times, so that, per horizontal unit of length of the dryer section in the machine direction, by means of the modules 10 1 and 10 2 , quantities of water larger than average can be evaporated, i.e. the intensity of drying can be increased by these means in the initial part of the dryer section. In this way, in the modules 10 1 and 10 2 , the dry solids content and the strength of the web W to be dried can be raised rapidly to an adequate level so that, if necessary, it is also possible to start using free gaps in the subsequent stages of drying. Owing to the larger diameters D 1A , D 2A and D 3A of the cylinders 15A, and 21A, and of the rolls 22A, in the cylinders 15A and the rolls 22A, it is also possible to employ lower levels of negative pressure, which is of advantage in view of both the cost of equipment and the cost of energy. In the following table, a preferred exemplifying embodiment will be given concerning the dimensioning of the diameters D 1A ,D 2A ,D 3A ,D 1 ,D 2 , and D 3 of the cylinders 15A,21A,22A,15,21,22 shown in Fig. 7. D 1A 3.2 m D 2A 2.4 m D 3A 1.8 m D 1 2.4 m D 2 1.8 m D 3 1.5 m If the ratio of the cylinder diameters in the first drying modules 10 1 , 10 2 and in the following drying modules 10 3 , 10 4 is denoted with k, preferably k = D 1A / D 1 ≈ D 2A / D 2 ≈ D 3A / D 3 The ratio k mentioned above is chosen preferably in the range of k = 1.2...1.5, depending on the application and the paper quality. The cylinder diameters D 1 ,D 2 and D 3 in the latter drying modules 10 3 and 10 4 are chosen so that the dryer construction and the drying process are optimized both in view of the paper quality produced, the runnability, and the machine construction, in view of which, in the first modules 10 1 and 10 2 , substantially larger cylinder diameters D 1A , D 2A and D 3A are employed, for the reasons given above. According to Fig. 8, the drying modules 10 1 ,10 2 ,10 3 and 10 4 are followed by a prior-art group R 3 with single-wire draw, whose drying wire is denoted with the reference 20 3 , the contact-drying cylinder in the upper row with the reference 21a, and the reversing suction rolls in the lower row with the reference 22a. According to Fig. 8, as the last group RTW N in the dryer section, a group with twin-wire draw in itself known is used, in which the web W has free unsupported draws W o between the rows of contact-drying cylinders 21c and 21d. In the group RTW N with twin-wire draw, there is an upper wire 20c, which is guided by guide rolls 22c fitted in the gaps between the drying cylinders 21c, and a corresponding lower wire 20d, which is guided by guide rolls 22d fitted in the gaps between the drying cylinders 21d in the lower row. The dryer section shown in Fig. 8 is particularly well suitable for modernization of existing dryer sections, so that the groups R 3 ...R n with single-wire draw and/or the group RTW N with twin-wire draw are horizontal groups in the final end of the dryer section to be modernized and the old groups in the initial end are replaced by the drying modules 10 1 ,10 2 ,10 3 and 10 4 , by whose means the drying capacity and the runnability can be increased so that the web speed in the dryer section can be increased to the level required by the modernization of the paper machine. The concept shown in Fig. 8 can also be applied so that the groups R 3 ...R n and/or RTW N are substituted for by one or several drying modules 10 5 ...10 N . In the dryer section in accordance with the invention, it is also possible to provide an arrangement for the control and regulation of the transverse drying profile of the paper. This can be accomplished so that one or several drying modules 10 are provided with such a hood 11 for a blowing-on drying cylinder 15 as is divided into several blocks in the transverse direction of the machine, preferably by means of vertical partition walls placed in the machine direction (not shown). Into said blocks, drying gases of different temperature, humidity, and/or pressure, as compared with one another, are passed. Instead of this, or in addition to this, in different blocks, it is possible to employ sets of drying-gas jets of different velocities. By means of this arrangement, the drying of the paper web W can be regulated in the transverse direction, and the paper web can be given a transverse moisture profile of exactly the desired form, usually uniform. The realization of said regulation from block to block in the control of the transverse moisture profile is in itself known from various connections, so that it will not be described in more detail in this connection, nor illustrated in the figures. In the following, a simulation example will be given in the form of a table concerning the evaporation capacities inside a drying module when through-drying is not employed on the large cylinder 15. In the following table, column a) gives the evaporation capacities expressed as the units kg H 2 O/h (kilograms of H 2 O per hour) in the initial end of the dryer section, and column b) gives the corresponding evaporation capacities in the final end of the dryer section. Moreover, the drying capacities of the different parts of the module are, in the following table, also given as percentages out of the total evaporation capacity of the module 10. a) b) kg H 2 O/h % kg H 2 O/h % large cylinder (15) 4429.7 67.7 4884.1 76.1 1st contact cyl. (21) 544.7 8.3 513.7 8.0 suction roll (22) 1140.9 17.5 671.6 10.5 2nd contact cyl. (21) 421.8 6.5 344.9 5.4 total 100.0 total 100.0 As comes out from the table above, of the entire evaporation capacity of the module 10, ∼ 65...75 % takes place on the large cylinder 15, while the rest of the evaporation capacity is divided substantially evenly between the pair of contact cylinders 21 and the reversing suction roll 22. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above by way of example only.

Claims: Dryer section of a paper machine, in particular of a high-speed paper machine, whose running speed is v ≈ 25...40 meters per second, wherein the dryer section comprises plural drying modules (10) placed one after the other, and wherein each of said drying modules (10) includes a loop of a drying wire (20) for supporting a web (W) to be dried, guide rolls (25) for guiding the drying wire, two smooth-faced (21') heated contact-drying cylinders (21) placed outside said drying-wire loop, a large-diameter blowing-on-drying cylinder (15), which is placed inside the drying-wire loop and has a diameter D 1 > 2m, wherein said blowing-on-drying cylinder (15) has an outer mantle (16) which is provided with grooves (16R) and/or perforations (16P), and wherein the web (W) and the drying wire (20) are passed over a contact sector a on said blowing-on-drying cylinder (15) which is a > 180°, and a drying hood (11) provided on the contact sector a of said blowing-on-drying cylinder (15), in the interior of which hood (11), at the proximity of the outer face of the web (W), there is a nozzle field (43) through which a set of drying-gas jets can be applied at a high velocity against the free outer face of the web (W) in a substantial area of said contact sector a, wherein, at the proximity of said blowing-on-drying cylinder (15), at both sides thereof, one of said two contact-drying cylinders (21) is placed, and wherein said contact-drying cylinders (21) have a diameter D 2 < D 1 , characterized in that a reversing suction roll (22) having a diameter D 3 < D 2 is placed, in the running direction of the web (W), before or after said contact-drying cylinders (21) and inside the drying-wire loop, in that the web (W) and the drying wire (20) are passed over a contact sector b on said contact-drying cylinders (21) which is b > 180°, and in that , in the initial end of the dryer section, in one or several, preferably at least two, initial drying module(s) (10 1 , 10 2 ) of said plural drying modules (10), larger diameters D 1A , D 2A and/or D 3A of said blowing-on-drying cylinder (15A), said contact-drying cylinders (21A) and/or said reversing suction roll (22A) are employed as compared with the corresponding diameters D 1 , D 2 and/or D 3 employed in said drying module (s) (10 3 , 10 4 ) of said plural drying modules (10) which follow(s) after said initial drying module (s) (10 1 , 10 2 ). Dryer section as claimed in claim 1, characterized in that the diameters D 1A , D 2A and/or D 3A in the initial drying module(s) (10 1 , 10 2 ) have been chosen so that their ratio k to the corresponding diameters D 1 , D 2 and/or D 3 in the following drying module(s) (10 3 , 10 4 ) is in the range of k = 1.2...1.5. Dryer section as claimed in claim 1 or 2, characterized in that , in addition to said plural drying modules (10), the dryer section includes one or several groups (R) of additional contact-drying cylinders (21a), which groups are preferably provided with single-wire draw. Dryer section as claimed in any of claims 1 to 3, characterized in that , in the rear end of the dryer section, there is/are one or several group(s) (RTW) with twin-wire draw, in which the paper web (W) has open free draws (W 0 ) between its rows of additional contact-drying cylinders (21c, 21d). Dryer section as claimed in any of claims 1 to 4, characterized in that the number of said drying modules (10) is N = 3...12. Dryer section as claimed in any of claims 1 to 5, characterized in that said diameters D 1 , D 2 and D 3 have been chosen so that D 1 /D 2 = 1.0...2.2, preferably D 1 /D 2 = 1.5...1.7, and D 2 /D 3 = 1.1...2.2, preferably D 2 /D 3 = 1.2...1.6, and/or that said contact-drying cylinders (21), said blowing-on-drying cylinder (15) and/or reversing suction roll (22) are placed, in the horizontal and vertical directions, in such a way in relation to one another and dimensioned in such a way that the horizontal distance between the centres of the two contact-drying cylinders (21) is l 3 = (0.3...2) • D 1 and the difference in height between the centres of the reversing suction roll (22) and the adjacent contact-drying cylinder (21) is h 2 = (0.1...1.1)• D 2 , and the difference in height between the centres of said contact-drying cylinder (21) and the blowing-on-drying cylinder (15) is h 1 = (2...10) • h 2 , preferably h 1 = (3...6) • h 2 . Dryer section as claimed in any of claims 1 to 6, characterized in that said blowing-on-drying cylinder (15) and/or as said reversing suction roll (22) is provided with a grooved outer mantle (16; 23) and with through perforations (16P) opening into the grooves, the interior of said cylinder and/or roll communicating with a source of negative pressure (37; 38) through a suction duct (18; 38a) placed in connection with an axle journal of the cylinder and/or roll. Dryer section as claimed in claim 6, characterized in that negative pressure is applied to the blowing-on-drying cylinders (15) and/or to the reversing suction roll (22) out of a suction box fitted inside the cylinder or roll and provided with seals, the negative pressure being applied to the contact sector a or a contact sector c on the reversing suction roll (22) that is covered by the web (W). Dryer section as claimed in any of claims 1 to 8, characterized in that the drying wires of two successive drying modules (10 3 , 10 4 ) are formed by a common drying wire (20 2 ), and that the horizontal distance between the centres of the blowing-on-drying cylinders (15) of the two successive drying modules is in the range of 1 11 ≈ (0.8...4) • D 1 . Dryer section as claimed in any of claims 1 to 9, characterized in that the drying wires of two or more successive drying modules (10 2 , 10 3 ) are formed by a common drying wire (20 2 ) and that, between said successive drying modules (10 2 , 10 3 ), there is an additional reversing suction roll (22), on which a sector c > 160° reverses the common drying wire (20 2 ) and the web (W) and on which the web (W) remains supported on the common drying wire (20 2 ) with the common drying wire (20 2 ) located between the additional reversing suction roll (22) and the web (W). Dryer section as claimed in any of claims 1 to 10, characterized in that in at least one of said drying modules (10) the two contact-drying cylinders (21) are placed underneath the blowing-on-drying cylinder (15). Dryer section as claimed in any of claims 1 to 11, characterized in that said successive drying modules (10) or pairs of drying modules having the common drying wire are inverted in such a way in relation to the adjacent module (10) or pair of drying modules that the side of the web (W) to be dried is changed when moving from one drying module or pair of drying modules to the next drying module or pair of drying modules. Dryer section as claimed in any of claims 1 to 12, characterized in that the drying hood (11) of one or several of said drying modules (10) is divided into two or more compartments (10a, 10b) in the running direction of the web (W). Dryer section as claimed in any of claims 1 to 13, characterized in that the drying hood (11) of one or several of said drying modules (10) is divided into several blocks in the direction transverse to the running direction of the web (W). The use of a dryer section as claimed in any of claims 1 to 14 for modernization of an existing dryer section, in particular in order to increase the running speed of the paper machine.

Inventors: HEIKKILAE PERTTI, ILMARINEN ANTTI, ILVESPAEAE HEIKKI, JOKIOINEN ILKKA, KARVINEN MIKKO, KORPELA MATTI, KUHASALO ANTTI, PARKER DICK, PETTERSSON HENRIK, SAILAS VAEINOE, TASKINEN PEKKA, YLI-KAUPPILA JOUKO, HEIKKILAE, PERTTI, ILMARINEN, ANTTI, ILVESPAEAE, HEIKKI, JOKIOINEN, ILKKA, KARVINEN, MIKKO, KORPELA, MATTI, KUHASALO, ANTTI, PARKER, DICK, PETTERSSON, HENRIK, SAILAS, VAEINOE, TASKINEN, PEKKA, YLI-KAUPPILA, JOUKO, Heikkilä, Pertti, Ilvespää, Heikki, Sailas, Väinö
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1994
Main classification: D21F5
IPC codes: D21F5